1. Field of the Invention
This invention relates to a method and apparatus for magnetooptically recording and retrieving data. For storing data, a suitable record medium is locally heated by a laser beam and cooled in the presence of an external magnetic field for changing the magnetic characteristics of the material of the record medium, and for reading data, those changes are detected with the aid of another laser beam in transmission through the record medium, by monitoring the rotation of the direction of polarization of the laser light, which rotation was caused by the recording of data.
2. Description of the Related Art
In present-day data processing, the storage of information is a key issue. Although the speed of processing the data has increased tremendously over the last decade, there is a demand for the ability to process ever larger volumes of data. The handling of large volumes of data, however, faces two related problems. The first problem is one of size: the storage facility for a large volume of data is big of necessity, and this brings about the second problem immediately, which is one of speed. The bigger the storage, the longer it takes to address the location where the desired information sits, and the longer become the paths the information has to travel from its storage location to a processing station. Obviously, storage design must strive at shrinking memory space as much as possible, in other words, aim at increasing storage density.
The highest so-far-reported data storage densities have been achieved with magneto-optic techniques. These are based on the unique properties of rare earth/transition metal thin films having a sufficiently large perpendicular anisotropy to stabilize vertical magnetic domains. The information is stored by way of selectively aligning the direction of magnetization of those domains, and the information is retrieved by monitoring the alignment of the magnetization at the selected location.
The writing of bits of information is performed by heating tiny spots of the storage medium with a well-focused high-intensity laser beam to temperatures above the Curie (or compensation) temperature T.sub.c of the material used for the storage medium, in the presence of a magnetic field. At the Curie temperature, the material loses its spontaneous or previous remanent magnetization, and its magnetic dipoles can assume the direction of an external field, which they retain after the laser beam, i.e. the source of heat, is turned off.
In prior art magneto-optic storage devices, the information is read by shining a low-intensity laser beam onto the addressed storage location and analyzing the rotation of the polarization of the reflected light induced by the magneto-optic Kerr effect. (Kerr effect is the name for the phenomenon that linearly polarized light when reflected from magnetic material becomes slightly elliptically-polarized, with the angle of rotation of the polarization direction being proportional to the magnetization.)
U.S. Pat. No. 4,823,220 describes a detector for detecting the p- and s-components of the light reflected by the magneto-optic storage medium, wherein the p- and s-components are converted from polarization rotations into a single combined-intensity modulated beam which is then intensity modulation detected for indicating the information content of the beam. In doing so, first either the p- or s-component is rotated to the s- or p-polarization plane, respectively, then the rotated and other light are processed using interferometric techniques to obtain either light or dark, which is intensity-modulated.
An interferometer particularly adapted to detect light received from a magneto-optic storage medium, i.e. reflected light which has rotated polarization caused by said storage medium, is also known from U.S. Pat. No. 4,823,220. It may include a polarization beam-splitting prism having a first face for receiving the reflected light from the medium, comprising p- and s-polarization components, and for directing these components on first and second perpendicular light paths. The first path carries the one component in a first minimal delay, the second path carries the second component in a second, equal or greater delay. The light paths are then combined for creating an interference pattern from which the optical relationship between said first and second components can be determined and converted into a modulated electrical signal carrying a representation of the stored information.
In conventional magneto-optic storage devices, the ability to provide high-density recording on optical disks requires high laser power which has been available only at comparatively long wavelengths, approximately 800 nm. To increase the recording speed, one has used 40 mW injection lasers which turned out, however, to have a limited life cycle. Semiconductor laser devices have been developed which produce shorter wavelengths, though with low efficiency, such that at 400 nm the incident power will be less than 1 mW. The shorter wavelength means, however, that the bit diameter can be cut in half, thereby increasing the density by four times.
Representative of the prior art devices making use of these considerations is the article "High-Density Optical Disk Recording System", IBM Technical Disclosure Bulletin, Vol. 31, No. 11 (1989) pp. 157-159. In accordance with the technique described in this reference, high-density recording of data is obtained by utilizing a recording head containing a 1 mW laser of 400 nm wavelength and a 5 mW laser of 800 nm wavelength. The light from both lasers passes through a common lens system designed to also serve for collecting the light reflected from the recording medium for read-out purposes. For writing information, both lasers are activated for selective cooperation with photoconductor layers provided on the storage medium.
The techniques of magneto-optic recording reported in the prior art are capable of writing domains with diameters well below the wavelength .lambda. (at present &lt;200 nm) since the laser intensity can be chosen such that only the central part of the Gaussian beam profile locally heats the recording medium above its Curie or compensation temperature T.sub.c and, hence, allows switching of magnetic domains. This technique is really very efficient since a small change in laser intensity produces a small change in temperature, which--in the neighborhood of T.sub.c --is sufficient to produce a large change in the magnetic anisotropy, and this determines the domain switching characteristics. In the prior art system, magneto-optic reading is, however, diffraction-limited to .lambda./2 because the detection uses the Kerr effect in the far-field. In fact, the achievable resolution is &gt;1 .mu.m (even worse), owing to the laser beam widening because of imperfections in the laser diodes and optics.